1. Field of the Invention
The present invention relates to a device and method for controlling an air-fuel ratio of an internal combustion engine, in which an air-fuel ratio of a combustion mixture is controlled based on an oxygen quantity stored in a catalytic converter.
2. Related Art of the Invention
Heretofore, there is known an air-fuel ratio control device that estimates an oxygen quantity stored in a catalytic converter based on an air-fuel ratio to be detected by an oxygen sensor disposed upstream of the catalytic converter and an intake air quantity of an engine, to control an air fuel ratio of the combustion mixture so that the stored oxygen quantity reaches a target value (refer to Japanese Unexamined Patent Publication Nos. 6-249028, 10-184425).
The oxygen quantity stored in the catalytic converter can be accurately estimated from the intake air quantity when the engine is in steady operation. When the engine is in transient operation, however, since a change in exhaust gas quantity flowing into the catalytic converter is delayed behind a change in intake air quantity, there occurs an estimation error in the oxygen quantity stored in the catalytic converter leading a possibility of deterioration in the control precision of air-fuel ratio.
It is therefore an object of the present invention to provide a device and method for controlling an air-fuel ratio of an internal combustion engine which is capable of estimating an oxygen quantity stored in a catalytic converter based on an intake air quantity with high precision and, hence, of maintaining the control precision of air-fuel ratio.
In order to accomplish the above object, according to the present invention, a detection value of the intake air quantity is corrected based on transient operation conditions, and the oxygen quantity stored in the catalytic converter is estimated based on the above corrected intake air quantity and an oxygen concentration in the exhaust.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.